This invention relates to a top or bottom structure of a square cylindrical container formed of board paper having thermal adhesive plastic layers on both front and back surfaces.
FIG. 4 shows the construction of a conventional paper container proposed by Japanese Utility Model Publication No. 2205/1984. This square cylindrical paper container is constructed with a flat type top or bottom having opposed outside bottom plates 36 and 38 formed on board paper having thermal adhesive plastic layers on both front and back surfaces, and opposed inside bottom plates 37 and 39 having folding lines of isosceles triangular shape. End pieces extend from the respective bottom plates. The end piece 40 of the first outside bottom plate 36, which is disposed at the outermost side during the assembling and shaping time, extends further than the other end pieces 41, 42 and 43, the end piece 42 of the second outside bottom plate being cut out at the right and left ends, the end pieces 41, 43 of the inside bottom plates being cut out only 2/5 of the entire length of the end pieces at the end of the side adjacent to the second outside bottom plate, and the remaining end pieces 41, 42 and 43 being held by the inner surface of the end piece 40 of the first outside bottom plate and the outer surface of the second outside bottom plate and being thermally bonded.
The conventional paper container of the above mentioned construction has strong impregnability and is capable of holding a content for long periods of time. In this container, the end piece 42 of the second outside bottom plate is cut out at the right and left side ends, the end pieces 41 and 43 of the inside bottom plates are cut out only 2/5 of the entire length of the pieces at the end of the side adjacent to the second outside bottom plate, and the portion which does not directly contribute to the sealability is suitably cut out to provide the end pieces, thereby weakening the resistance of the bending portion and further enhancing the sealability of the opposed portions of the top of the inside bottom plates.
The construction of the abovementioned conventional paper container fundamentally provides excellent sealing, but excessive load is applied to the end piece 42 of outside bottom plate 38. More particularly, during assembling and shaping of the paper container, the outside end piece 42, the end pieces 41 and 43 of the inside bottom plates and the end piece 40 of outside bottom plate 36 are folded entirely to become flat, superposed and heat sealed. At this time, the end piece 42 is folded at 180.degree., the end pieces 44 and 45 of the sides having cutouts of the end pieces 41 and 43 are also similarly folded at 180.degree. so as to be superposed on the end piece 42, and a flat bottom and top are obtained by completely folding them. However, since folding seams 46 and 47, formed when the end pieces 44 and 45 are folded, are disposed along the line of the bottom, folding line 48 of the end piece 42 is displaced slightly inwardly of the outer bottom plate 38 from the bottom line.
Therefore, when the folding line 48 of the end piece 42 is disposed on the same line as the folding seams 46 and 47, both the pressing perpendicular force for flattening the bottom and the pressing horizontal force for pressing toward the direction of the outside bottom plate 38 are applied to the end piece 42. Thus, cracks are formed at the right and left side roots of the end piece 42, thereby sometimes causing leakage of the liquid in the container.